安卓：在画布中滚动位图

Question

我想创建一个视图，其中包含一个大的位图(比屏幕大)，放置在画布上。这样用户就可以滚动Bitmap并在其上绘制手指绘画。我正在遵循来自Android API演示中的FingerPaint代码。

但我无法使用Canvas产生滚动的Bitmap。增加位图大小似乎不起作用。

我尝试将视图类添加到ScrollView中，但是视图的onDraw()没有被调用，我得到了一个空白屏幕。

请帮助我解决这个问题。谢谢。

- sugandhs

Answer 1

尝试制作一个自定义视图，像这样：

public class ScrollableImage extends View {
    private Bitmap bmLargeImage; // bitmap large enough to be scrolled
    private Rect displayRect = null; // rect we display to
    private Rect scrollRect = null; // rect we scroll over our bitmap with
    private int scrollRectX = 0; // current left location of scroll rect
    private int scrollRectY = 0; // current top location of scroll rect
    private float scrollByX = 0; // x amount to scroll by
    private float scrollByY = 0; // y amount to scroll by

    private int width, height;

    private Paint background;

    public ScrollableImage(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public ScrollableImage(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }

    public void setSize(int width, int height) {
        background = new Paint();
        background.setColor(Color.WHITE);
        this.width = width;
        this.height = height;

        // Destination rect for our main canvas draw. It never changes.
        displayRect = new Rect(0, 0, width, height);
        // Scroll rect: this will be used to 'scroll around' over the
        // bitmap in memory. Initialize as above.
        scrollRect = new Rect(0, 0, width, height);
        // scrollRect = new Rect(0, 0, bmp.getWidth(), bmp.getHeight());
    }

    public void setImage(Bitmap bmp) {
        if (bmLargeImage != null)
            bmLargeImage.recycle();

    bmLargeImage = bmp;
        scrollRect = new Rect(0, 0, width, height);
        scrollRectX = 0;
        scrollRectY = 0;
        scrollByX = 0;
        scrollByY = 0;
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        return true; // done with this event so consume it
    }

    public void notifyScroll(float distX, float distY) {
        scrollByX = distX; // move update x increment
        scrollByY = distY; // move update y increment
    }

    @Override
    protected void onDraw(Canvas canvas) {

        if (bmLargeImage == null)
            return;

        if (scrollByX != 0 || scrollByY != 0) {
            // Our move updates are calculated in ACTION_MOVE in the opposite     direction
            // from how we want to move the scroll rect. Think of this as
            // dragging to
            // the left being the same as sliding the scroll rect to the right.
            int newScrollRectX = scrollRectX - (int) scrollByX;
            int newScrollRectY = scrollRectY - (int) scrollByY;
            scrollByX = 0;
            scrollByY = 0;

            // Don't scroll off the left or right edges of the bitmap.
            if (newScrollRectX < 0)
                newScrollRectX = 0;
            else if (newScrollRectX > (bmLargeImage.getWidth() - width))
                newScrollRectX = (bmLargeImage.getWidth() - width);

            // Don't scroll off the top or bottom edges of the bitmap.
            if (newScrollRectY < 0)
                newScrollRectY = 0;
            else if (newScrollRectY > (bmLargeImage.getHeight() - height))
                newScrollRectY = (bmLargeImage.getHeight() - height);
            scrollRect.set(newScrollRectX, newScrollRectY, newScrollRectX
                    + width, newScrollRectY + height);

            scrollRectX = newScrollRectX;
            scrollRectY = newScrollRectY;
        }

        canvas.drawRect(displayRect, background);
        // We have our updated scroll rect coordinates, set them and draw.
        canvas.drawBitmap(bmLargeImage, scrollRect, displayRect, background);

    }
}

在手势监听器中，我有一个onScroll的实现

其中img是您的ScrollableImage实例。

记得使用setImage设置您的大图像。还要使用setSize设置显示的大小。

public boolean onScroll(MotionEvent e1, MotionEvent e2,
            float distanceX, float distanceY) {
            img.notifyScroll(-distanceX, -distanceY);
            img.invalidate();
        return true;
    }

在此链接中，您可以找到一个示例，演示如何使用BitmapRegionDecoder API。该示例包含一张大型的（6000,4000）地球图像，用户可以以全分辨率滚动浏览。

- K_Anas

---

感谢K_Anas的回答。但是我不应该使用任何外部图像作为位图。位图必须由Bitmap.createBitmap（width，height，Bitmap.Config.ARGB_8888）创建，并在调用onDraw（Canvas canvas）时绘制在画布上。就像API Demos中的FingerPaint示例一样。但它应该是可滚动的。 - sugandhs

@K_Anas，如果位图很大，上面的代码很可能会耗尽内存，对吗？那么您是否建议使用位图区域解码器或者有没有更简单的方法可以实现您的方法？ - Snake